Hi Leland,
The root cause is the much higher thermal expansion mismatch on cooling from reflow.
It comes from a number of negative factors coming into play:
(1) SAC solders are significantly stiffer (greater modulus of elasticity) than SnPb;
(2) SAC solder are significantly stronger (greater yield strength);
(3) SAC solders creep much slower (therefore provide no 'give');
(4) SAC solder have a Solidus of 217°C instead 183°C, thereby almost doubling the delta-T to the PCB glass transition T;
(5) SAC solders, because (1-3) become a mechanically significant member with a large CTE on top of the copper traces;
(6) the PCB base materials in order to be RoHS-capable are typically phenolic-, not dicy-, cured and contain fillers—this can reduce the cohesive strength of the resin matrix (e.g., pad cratering);
(7) the higher T's tend to reduce the peel strength of the copper traces on the laminate;
(8) in many cases the cooling rates from reflow have remained the same as with SnPb [as I mentioned earlier, HP has mandated that cooling rates need to be reduced to 1.5°C/sec or lower) setting up significant thermal gradients, thermal expansion [of course with cooling, this should read 'contraction'] mismatches, and much higher stress levels than were possible with SnPb;
(9) most reflow ovens are not designed for achieving cooling rates of 1.5°C/sec or less; for anything but 10-zone ovens, additional cooling zones need to be added [at 1.5°C/sec you need a cooling section of about 100 cm, at 1.0°C/sec that becomes 150 cm];
(10) and there are likely to be other factors as well.
I think/hope your curiosity has been satsified.
Werner
-----Original Message-----
From: Leland Woodall <[log in to unmask]>
To: [log in to unmask]
Sent: Wed, Feb 3, 2010 9:57 am
Subject: Re: [TN] track lifting
Brian,
Was the strain you mention below originate from the reflow process? Was
it because of a thermal mismatch or an accelerated cooling rate?
Just curious...
Leland
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Chandler
Sent: Wednesday, February 03, 2010 7:28 AM
To: [log in to unmask]
Subject: Re: [TN] track lifting
We just conducted a study on acceptable strain levels for lead free
products.
Our BGA test coupons showed this exact result from excess strain.
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